Image forming apparatus and storage medium for high-accuracy colorimetry

ABSTRACT

An image forming apparatus that prints an adjustment pattern on a sheet includes a generator and an image former. The adjustment pattern is for correcting a color value obtained by reading a second patch region on the sheet with a second reader based on a color value obtained by reading a first patch region on the sheet with a first reader. The generator generates image data of the adjustment pattern, and the image former performs printing based on the image data. The adjustment pattern includes a first patch printed in the first patch region and a second patch printed in a region other than the first patch region in the second patch region. The second patch includes a common color patch in a same color as a color of the first patch, and a size of the first patch is larger than a size of the second patch.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims prioritybenefit to, U.S. patent application Ser. No. 15/722,537, filed Oct. 2,2017, which claims priority to Japanese Patent Application No.2016-196090, filed Oct. 4, 2016. Each of the above referencedapplications is herein incorporated by reference in their entirety.

BACKGROUND 1. Technological Field

The present invention relates to an image forming apparatus and astorage medium.

2. Description of the Related Art

In image forming apparatuses, there has been recently used a techniqueof setting an image reading device such as a line sensor on a sheetconveyance path of a sheet after image formation, reading color patcheson the sheet by the image reading device and feeding back the readingresults to an image forming condition for performing image outputstably.

Calibration of the image reading device has also been performed bymeasuring common color patches with a spectrophotometer and the imagereading device. For example, there has been used a technique ofobtaining reading results of patches which can be read by both of acolorimeter that reads a partial region in a main scanning direction anda scanner that reads over the image forming width in the main scanningdirection, obtaining errors between the reading results by the scannerand the reading results by the colorimeter, feeding back the errors ascorrection values and thereby improving measurement accuracy of thepatches that were read by only the scanner (not measured by thecolorimeter) (see Japanese Patent Application Laid-Open Publication No.2015-226128).

By associating the output values of the scanner with the output valuesof the colorimeter, colorimetry can be performed efficiently by usingthe scanner.

However, even if a same color is read, the RGB value obtained byreading, with the scanner, a patch included in the reading region foronly the scanner is possibly different from the RGB value obtained byreading, with the scanner, a patch included in the common reading regionfor the colorimeter and the scanner due to the influence of flare andirregularities in the surface of the scanner.

For example, when the output value (average pixel value) of green by thescanner is “230/255” with respect to a white patch (white portion of thesheet) which is included in the common reading region, there are somecases where, due to the influence of flare, the output value of green bythe scanner is “200/255” with respect to a white patch (in the samecolor as the white patch included in the common reading region) which isincluded in the reading region for only the scanner, and the density isdetected to be higher.

In this case, when the scanner is adjusted to the colors measured by thecolorimeter on the basis of the output value for the common readingregion, the obtained measurement value for the white patch included inthe reading region for only the scanner is a value indicating whitewhich is darker than actual.

In this way, the above conventional techniques have had a problem thatan error is caused in calibrating a scanner using a colorimeter and thatthe scanner cannot measure the colors accurately. When color correctionof the image forming apparatus is performed with the error,highly-accurate color correction cannot be achieved finally.

SUMMARY

The present invention has been made in consideration of the aboveproblems in the conventional techniques, and an object of the presentinvention is to perform colorimetry with high accuracy and highefficiency.

To achieve at least one of the abovementioned objects, according to anaspect of the present invention, an image forming apparatus reflectingone aspect of the present invention comprises: an image former whichforms an adjustment pattern on a sheet; a first reader which is disposedon a conveyance path of the sheet and outputs a first color value byreading the adjustment pattern, and a second reader which is disposed onthe conveyance path of the sheet and outputs a second color value thatis different from the first color value by reading the adjustmentpattern; and a hardware processor, wherein the first reader reads apartial region in a sheet width direction, the second reader is capableof reading an entire region in the sheet width direction, the adjustmentpattern has a first patch region in which a patch that is readable bythe first reader is arranged and a second patch region in which a patchthat is readable by only the second reader is arranged, the second patchregion includes at least a common color patch which is a patch in a samecolor as a color of the patch included in the first patch region, thehardware processor obtains an estimate first color value by converting asecond color value of each patch in the second patch region which isoutput by the second reader into a first color value, the hardwareprocessor obtains a correction value of the estimate first color valuefor each of the common color patch by comparing a first color value ofeach patch in the first patch region which is output by the first readerwith the estimate first color value obtained from the common color patchin the second patch region corresponding to each patch in the firstpatch region, and the hardware processor corrects the estimate firstcolor value obtained from every patch in the second patch region basedon the correction value of the estimate first color value obtained foreach of the common color patch.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features provided by one or more embodiments of theinvention will become more fully understood from the detaileddescription given hereinafter and the appended drawings which are givenby way of illustration only, and thus are not intended as a definitionof the limits of the present invention, and wherein:

FIG. 1 is a schematic sectional view of an image forming apparatus in afirst embodiment of the present invention;

FIG. 2 is a block diagram showing a functional configuration of theimage forming apparatus;

FIG. 3 is an example of a color chart in which an adjustment patternincluding a plurality of patches is formed;

FIG. 4 is a view schematically showing the relationship between acolorimeter, a scanner and a color chart;

FIG. 5 is a view for explaining the summary of the present invention;

FIG. 6 is a flowchart showing image adjustment processing; and

FIG. 7 is an example of a patch group included in a second patch regionof a color chart used by an image forming apparatus in a secondembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one or more embodiments of the present invention will bedescribed with reference to the drawings. However, the scope of theinvention is not limited to the disclosed embodiments.

First Embodiment

First, a first embodiment of an image forming apparatus according to thepreset invention will be described.

FIG. 1 is a schematic sectional view of an image forming apparatus 100in the first embodiment.

The image forming apparatus 100 includes an image forming section 10, animage reading section 20 and such like.

The image forming section 10 is for electrophotographic image formationand forms an image on a sheet P on the basis of image data correspondingto respective colors of yellow (Y), magenta (M), cyan (C) and black (K).The image forming section 10 includes photoreceptor drums 1Y, 1M, 1C and1K, charging sections 2Y, 2M, 2C and 2K, exposing sections 3Y, 3M, 3Cand 3K, developing sections 4Y, 4M, 4C and 4K, primary transferringrollers 5Y, 5M, 5C and 5K, an intermediate transfer belt 6, a secondarytransfer roller 7, a fixing section 8, sheet feeding sections 9 and suchlike.

A yellow toner image is formed on the photoreceptor drum 1Y. Thecharging section 2Y uniformly charges the photoreceptor drum 1Y. Theexposing section 3Y scans and exposes the surface of the photoreceptordrum 1Y with laser beams to form an electrostatic latent image on thebasis of the yellow image data. The developing section 4Y attachesyellow toner to the electrostatic latent image on the photoreceptor drum1Y to perform development.

The same processing is performed with respect to the other colors ofmagenta, cyan and black.

The toner images of respective colors formed on the photoreceptor drums1Y, 1M, 1C and 1K are sequentially transferred (primary transfer) ontothe intermediate transfer belt 6 by the primary transfer rollers 5Y, 5M,5C and 5K. That is, color toner images formed of overlapping four colortoner images are formed on the intermediate transfer belt 6.

The color toner images on the intermediate transfer belt 6 aretransferred all at once onto one surface of the sheet P, which was fedfrom a sheet feeding section 9, by the secondary transfer roller 7(secondary transfer).

The fixing section 8, which includes a heating roller heating the sheetP on which the color toner images are transferred and a pressing rollerpressing the sheet P, fixes the color toner images onto the sheet P byheating and pressing.

In a case of forming images on both sides of the sheet P, the sheet P isconveyed to a conveyance path R1, both the sides are turned over, andthereafter the sheet P is conveyed to the secondary transfer roller 7again.

The image reading section 20 includes a colorimeter 30 as a first readerand a scanner 40 as a second reader.

The colorimeter 30 and the scanner 40 are provided so as to be close toeach other on a sheet conveyance path in the downstream side of thefixing section 8 in the conveyance direction of the sheet P. Thecolorimeter 30 and the scanner 40 can read inline (on the conveyancepath in the apparatus) an adjustment pattern which is formed on thesheet P.

The colorimeter 30 is a spectrophotometer which detects spectralreflectivity of the image formed on the sheet P for each wavelength andmeasures the colors of the image. The colorimeter 30 outputs a Lab valueas a first color value to a controller 11 (see FIG. 2). The Lab valuerepresents a color with a numerical value by a combination of (L*, a*,b*) in the L*a*b* color space. The colorimeter 30 can read out only apartial region in the sheet width direction (direction which isorthogonal to the conveyance direction of sheet P and parallel to thesheet surface).

The scanner 40 is a line sensor which has CCDs (Charge Coupled Devices)that are arranged in lines over the entire sheet width direction, andreads out a one-dimensional image. By performing a reading operation inaccordance with the timing when the sheet P having an image formedthereon is conveyed, the scanner 40 obtains a two-dimensional imageformed on the sheet P. That is, the scanner 40 can read out the entireregion in the sheet width direction. The scanner 40 outputs an RGB valueas a second color value having a tone value of each color to the CPU 11(see FIG. 2) for each channel of red (R), green (G) and blue (B). TheRGB value represents a color with a numerical value by a combination of(R, B) in the RGB color space.

The scanner 40 may be a camera which captures a two-dimensional image.

FIG. 2 is a block diagram showing a functional configuration of theimage forming apparatus 100.

As shown in FIG. 2, the image forming apparatus 100 is configured byincluding the controller 11, a storage section 12, an operation section13, a display section 14, a communication section 15, a densityirregularity correction section 16, a color correction section 17, animage forming section 10, an image reading section 20 and such like. Asfor the configurations which have been already explained, theexplanation is omitted.

The controller 11 is configured by including a CPU (Central ProcessingUnit), a RAM (Random Access Memory) and such like, and integrallycontrols the operations of the sections in the image forming apparatus100 in accordance with various processing programs stored in the storagesection 12.

The storage section 12 is formed of a hard disk, a flash memory and suchlike, and stores various processing programs and various types of datasuch as parameter and files necessary for executing the programs. Forexample, image data of an adjustment pattern including a plurality ofcolor patches is stored in the storage section 12. The patch positionand the CMYK value forming a patch are stored in the storage section 12so as to be associated with each other for each patch included in theadjustment pattern.

The operation section 13 includes a touch panel which is formed so as tocover the display screen of the display section 14 and various operationbuttons such as numeric buttons and a start button, and outputs anoperation signal based on user's operation to the controller 11.

The display section 14 is configured by including an LCD (Liquid CrystalDisplay), and displays various screens in accordance with theinstruction of a display signal input from the controller 11.

The communication section 15 transmits and receives data to and from anexternal device connected to a communication network such as a LAN(Local Area Network).

The density irregularity correction section 16 corrects densityirregularity in the sheet width direction in the image forming section10. For example, the density irregularity correction section 16 correctsthe density irregularity by performing image processing to image datawhich is a target of image formation in the image forming section 10.Alternatively, the density irregularity correction section 16 maycorrect the density irregularity by adjusting the intensities of writingbeams emitted from the exposing sections 3Y, 3M, 3C, 3K.

The color correction section 17 corrects the colors of an image to beformed by the image forming section 10 on the basis of a colorcorrection parameter. In the image forming apparatus 100, target Labvalues are determined with respect to CMYK values in advance, and thecolor correction section 17 calculates the color correction parameter sothat the colors of the image formed by the image forming section 10 arethe target Lab values.

The controller 11 controls the image forming section 10 to form theadjustment pattern including a plurality of color patches on the sheetP. When the adjustment pattern is formed by the image forming section10, the density irregularity correction section 16 is applied.

The controller 11 obtains output values obtained by reading theplurality of patches included in the adjustment pattern from each of thecolorimeter 30 and the scanner 40.

FIG. 3 is an example of the color chart 50 in which the adjustmentpattern including a plurality of patches is formed. FIG. 4 schematicallyshows the relationship between the colorimeter 30, the scanner 40 andthe color chart 50.

The color chart 50 includes a first patch region 51 in which patchesreadable by the colorimeter 30 are arranged and second patch regions 52in each of which patches readable by only the scanner 40 are arranged.The second patch regions 52 are located at positions which thecolorimeter 30 cannot measure. The patches are respectively formed invarious colors of CMYK values.

Since the patch included in the first patch region 51 needs to have asize which can be stably measured by the colorimeter 30, the patchincluded in the first patch region 51 is larger than the patch includedin the second patch regions 52. On the other hand, the patch included inthe second patch regions 52 may have a size smaller than the patchincluded in the first patch region 51. By using patches in a small sizein the second patch regions 52, many patches can be arranged in thesecond patch regions 52.

The second patch regions 52 include at least a common color patch whichis a patch in a same color (patch defined by a same CMYK value) as apatch included in the first patch region 51. That is, the patches insame colors as the patches included in the first patch region 51 arenecessarily included in the second patch region 52.

The patches in the second patch regions 52 are randomly arranged. Thatis, the colors of the patches are not arranged in a regular order in thesecond patch regions 52. It is desirable that the arrangement of thepatches in the second patch regions 52 is determined by using a randompattern (blue noise and such like) with a high dispersibility.

The second patch regions 52 include a plurality of patches in a samecolor for a common color patch. For example, as shown in FIG. 3, thepatch P1 included in the first patch region 51 and the patches P2 and P3included in the second patch regions 52 are in the same color.

In the second patch regions 52, at least one of the patches around acommon color patch is a patch in a color different from the color of thecommon color patch.

Here, the summary of the present invention will be described withreference to FIG. 5.

The controller 11 obtains Lab values (measured Lab values) from thecolorimeter 30, the Lab values being obtained by reading the patchesincluded in the first patch region 51 in the color chart 50. The numberof patches included in the first patch region 51 is h.

The controller 11 obtains RGB values from the scanner 40, the RGB valuesbeing obtained by reading the patches included in the second patchregions 52 in the color chart 50. The number of patches included in thesecond patch regions 52 is j (j>h).

The controller 11 performs color conversion of the RGB values for therespective patches in the second patch regions 52 output from thescanner 40 into the Lab values, and obtains estimate Lab values afterthe conversion. That is, the estimate Lab value corresponding to each ofthe j patches is obtained.

The controller 11 extracts data of the common color patches (h patches)from among the estimate Lab values corresponding to respective patchesin the second patch regions 52.

The controller 11 compares measured Lab values of the patches in thefirst patch region 51 which were output from the colorimeter 30 with theestimate Lab values obtained from the common color patches in the secondpatch regions 52 corresponding to the respective patches in the firstpatch region 51, and obtains correction values of the estimate Labvalues for the respective common color patches. Specifically, inaccordance with the formula (1), the controller 11 obtains, as thecorrection value of the estimate Lab value, the difference Labvalue_(offset) which is the difference between the measured Lab valueobtained from the colorimeter 30 and the estimate Lab value obtainedfrom the common color patch in the second patch region 52.

[Expression 1]

Lab VALUE_(offset)=MEASURED Lab VALUE−ESTIMATE Lab VALUE  (1)

When the difference Lab value offset is obtained, the difference isobtained between L* values for the L* value, between a* values for thea* value and between b* values for the b* value. That is, the Lab valueoffset is also formed of the combination of three values.

The controller 11 corrects the estimate Lab values obtained from all thepatches in the second patch regions 52 on the basis of the correctionvalues of the estimate Lab values for the respective common colorpatches. The estimate Lab values which were corrected are referred to ascorrected estimate Lab values. The corrected estimate Lab values areobtained from the formula (2).

$\begin{matrix}{\mspace{79mu} \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack} & \; \\{{{CORRECTED}\mspace{14mu} {ESTIMATED}\mspace{14mu} {Lab}\mspace{14mu} {VALUE}} = {{{ESTIMATE}\mspace{14mu} {Lab}\mspace{14mu} {VALUE}} + {\sum\limits_{i = 1}^{N}{{Coef}_{I}*{Lab}\mspace{14mu} {VALUE}_{{offset},\; i}}}}} & (2)\end{matrix}$

Here, the Lab value_(offset,i) is the Lab value offset for i-th patch(i=1 to N) (the patch which has a same color in the first patch region51 and the second patch regions 52), and the Coef_(i) is the weightingcoefficient for the i-th patch. The sum of Coef_(i)s is 1.

In accordance with the weighting coefficient corresponding to thesimilarity with respect to the target color, the controller 11 adds thedifference Lab value offset for the common color patch to the estimateLab value of the target color and corrects the estimate Lab value. Thatis, with the weighting corresponding to the distance on the color range,the error is added to the estimate Lab value to calibrate the outputvalue of the scanner 40. The calculation of the corrected estimate Labvalue is also performed for each component of (L*, a*, b*). The RGBvalue is associated with the corrected estimate Lab value for each patchand the scanner profile is created.

The controller 11 controls the color correction section 17 to calculatethe color correction parameter on the basis of the corrected estimateLab values and correct the colors of an image to be formed by the imageforming section 10 on the basis of the color correction parameter.

Next, the operation of the image forming apparatus 100 in a firstembodiment will be described.

FIG. 6 is a flowchart showing image adjustment processing. Theprocessing is achieved by software processing in cooperation between thecontroller 11 and the program stored in the storage section 12.

First, when the user instructs formation of the adjustment pattern bythe operation from the operation section 13, the controller 11 readsimage data (CMYK value) of the adjustment pattern from the storagesection 12, and controls the image forming section 10 to form theadjustment pattern on the sheet P and output the color chart 50 (stepS1). When the adjustment pattern is formed, the controller 11 controlsthe density irregularity correction section 16 to correct the densityirregularity in the sheet width direction in the image forming section10.

Next, the controller 11 obtains Lab values (measured Lab values) whichwere obtained by reading the patches included in the first patch region51 of the color chart 50 from the colorimeter 30 (step S2). Thecontroller 11 stores the obtained measured Lab values in the storagesection 12 so as to be associated with the CMYK values for therespective patches.

Next, the controller 11 obtains RGB values which were obtained byreading patches included in the second patch regions 52 in the colorchart 50 from the scanner 40 (step S3). As the RGB value, the averagepixel value in each patch is used. The controller 11 stores the obtainedRGB values in the storage section 12 so as to be associated with theCMYK values for the respective patches. Data may be rearranged bysorting the obtained data in the order increasing the density of cyan,for example.

Next, the controller 11 obtains the estimate Lab values which wereobtained by converting the RGB values of respective patches in thesecond patch regions 52 output from the scanner 40 into the Lab values(step S4). A conversion table which was prepared in advance is used forconversion from the RGB values into the Lab values.

Next, the controller 11 compares the measured Lab value of each patch inthe first patch region 51 output from the colorimeter 30 with theestimate Lab value obtained from the common color patch in the secondpatch region 52 corresponding to the patch in the first patch region 51,and obtains the correction value (Lab value offset) of the estimate Labvalue for each of the common color patches in accordance with the aboveformula (1) (step S5).

In a case where the second patch regions 52 include a plurality ofpatches in a same color for a common color patch, the controller 11obtains the average value of the estimate Lab values obtained from theplurality of patches, and sets the average value as the estimate Labvalue of the common color patch. The controller 11 obtains thecorrection value of the estimate Lab value corresponding to the commoncolor patch on the basis of the average value of the estimate Labvalues.

Next, the controller 11 corrects the estimate Lab values obtained fromall the patches included in the second patch regions 52 on the basis ofthe correction values (Lab values offset) of the estimate Lab values forthe respective common color patches (step S6). Specifically, thecontroller 11 obtains the corrected estimate Lab values in accordancewith the above formula (2).

Next, the controller 11 associates the RGB value of each patch in thesecond patch regions 52 obtained in step S3 with the corrected estimateLab value (corrected estimate Lab value) of each patch obtained in stepS6 and creates a scanner profile for converting the RGB values into theLab values (step S7).

Next, the color correction section 17 calculates the color correctionparameter on the basis of the CMYK values corresponding to respectivepatches included in the second patch regions 52 and the correctedestimate Lab values (corrected estimate Lab values) corresponding torespective patches obtained in step S6 (step S8). As the colorcorrection parameter, there is used a parameter of a color profile, acalibration LUT (look up table), gamma correction and such like. Thecontroller 11 stores the color correction parameter in the storagesection 12.

Thereafter, the controller 11 controls the color correction section 17to correct the colors of an image to be formed by the image formingsection 10 on the basis of the calculated color correction parameter,and controls the image forming section 10 to perform image formation(step S9).

As described above, the image adjustment processing ends.

As described above, according to a first embodiment, patches in a samecolor which are arranged in different regions are respectively read bythe colorimeter 30 and the scanner 40, the Lab value output by thecolorimeter 30 is compared with the estimate Lab value obtained byconverting the RGB value output from the scanner 40, the correctionvalue of the estimate Lab value is obtained for each of the common colorpatches, and the estimate Lab values obtained from all the patches readby the scanner 40 are corrected on the basis of the correction values ofthe estimate Lab values which were obtained for the respective commoncolor patches. By enabling the conversion of the RGB value output fromthe scanner 40 into the corrected estimate Lab value, the scanner 40 canperform colorimetry with high accuracy and high efficiency. By dividingthe reading region (first patch region 51) of the colorimeter 30 fromthe reading region (second patch regions 52) of the scanner 40, it ispossible to obtain the RGB values obtained by reading the actual patchesfrom the scanner 40 regardless of the presence/absence of the influenceof flare and correctly associate the RGB values with the Lab valuesobtained from the colorimeter 30.

By making the size of the patch included in the first patch region 51larger than the patch included in the second patch regions 52, the patchincluded in the first patch region 51 can be measured stably by thecolorimeter 30. On the other hand, by making the size of patch includedin the second patch regions 52 small, many patches can be arranged inthe second patch regions 52, and calibration of the scanner 40 can beperformed accurately.

The patches in the second patch regions 52 are arranged randomly. In acase where the second patch regions 52 include a plurality of patches ina same color for a common color patch, the average value of the estimateLab values obtained from the plurality of patches is obtained, and theaverage value is set as the estimate Lab value of the common colorpatch. Thereby, stable reading data can be obtained from the scanner 40,the influence of flare and such like can be suppressed, and thus thereading accuracy can be improved.

In the first embodiment, the average value of the estimate Lab valuesobtained from the plurality of patches in the same color is obtained fora common color patch and set as the estimate Lab value of the commoncolor patch. However, the average value of the RGB values of theplurality of patches of a same color may be obtained for the commoncolor patch, and the value obtained by converting the average value ofthe RGB values into the Lab value may be set as the estimate Lab valueof the common color patch.

When the adjustment pattern is formed, the density irregularitycorrection section 16 corrects the density irregularity in the sheetwidth direction in the image forming section 10. Thus, stable readingdata can be obtained from the colorimeter 30 and the scanner 40 whilesuppressing the influence of the density irregularity.

The color correction section 17 calculates the color correctionparameter on the basis of the corrected estimate Lab values and correctsthe colors of an image to be formed by the image forming section 10 onthe basis of the color correction parameter. Thus, the reading result ofthe scanner 40 can be reflected in the image formation, and colorcorrection can be performed accurately.

Since at least one of the patches around the common color patch has adifferent color from the common color patch in the second patch regions52, calibration of the scanner 40 can be performed accurately in such astate that flare is generated at the time of reading by the scanner 40.

Second Embodiment

Next, the second embodiment to which the present invention is appliedwill be described.

Since the image forming apparatus in the second embodiment has a similarconfiguration as that of the image forming apparatus 100 shown in thefirst embodiment, FIGS. 1 and 2 are used and the illustration anddescription of the configuration will be omitted. The arrangement of thefirst patch region 51 and the second patch regions 52 in the color chartis also similar to the arrangement shown in FIGS. 3 and 4. Hereinafter,the configuration and the processing which are characteristic to thesecond embodiment will be described.

The controller 11 performs smoothing processing to estimate Lab valueswhich were obtained from each common color patch and the patches whichhave colors close to the common color patch in the second patch regions52, thereby determines the estimate Lab value corresponding to thecommon color patch, and obtains the correction value of the estimate Labvalue corresponding to the common color patch on the basis of theestimate Lab value after the smoothing processing. The color close tothe common color patch is a color which has a color value in apredetermined range from the common color patch when the color isrepresented by a color value in a color space.

Specifically, the controller 11 extracts estimate Lab values which wereobtained from the common color patch and the patches which have colorsclose to the common color patch included in the second patch regions 52,and obtains a moving average of the estimate Lab values along the RGBvalue obtained from each patch or the CMYK value when forming eachpatch.

The second patch regions 52 include a patch group 60 shown in FIG. 7,for example. The patch group 60 has 25 patches of 5×5, gradually changesthe density of cyan in one direction and gradually changes the densityof yellow in the direction orthogonal to the one direction. Though FIG.7 emphasizes the color difference between patches, the colors areactually within a close range. The central patch P11 in the patch group60 corresponds to the common color patch.

The controller 11 first obtains RGB values which were obtained byreading the respective 25 patches from the scanner 40, and obtains theestimate Lab values by converting the RGB values of the respectivepatches into the Lab values. The controller 11 performs smoothingprocessing of the estimate Lab values calculated for the respectivepatches. For example, an average value of the 25 estimate Lab values isobtained to determine the estimate Lab value corresponding to thecentral patch P11. A weighted average may be obtained according to thedegree of color differences between the central patch P11 and thesurrounding patches.

The operation in the second embodiment is nearly similar to the imageadjustment processing shown in FIG. 6.

However, in step S4, when the estimate Lab value of the common colorpatch in the second patch regions 52 is obtained, there is used a valueobtained by smoothing processing of the estimate Lab values obtainedfrom the central patch P11 and the respective patches (patches whichhave colors close to the common color patch) around the central patchP11 in the above-mentioned patch group 60.

In step S6, when the estimate Lab value of each patch other than thecentral patch P11 in the patch group 60 is corrected, the correctionvalue (Lab value offset) obtained for the central patch P11 is directlyadded to the estimate Lab value of the target patch. That is, thecorrected estimate Lab value of each patch other than the central patchP11 in the patch group 60 corresponds to the value calculated in theabove formula (2) in which the weighting coefficient of the centralpatch P11 is 1 and the weighting coefficient of each of the other commoncolor patches is 0.

As described above, according to the second embodiment, patches in asame color which are arranged in different regions are read by thecolorimeter 30 and the scanner 40, the Lab value output by thecolorimeter 30 is compared with the estimate Lab value obtained byconverting the RGB value output from the scanner 40, the correctionvalue of the estimate Lab value is obtained for each of the common colorpatches, and the estimate Lab values obtained from all the patches readby the scanner 40 are corrected on the basis of the correction values ofthe estimate Lab values for the respective common color patches. Byenabling conversion of the RGB values output from the scanner 40 intothe corrected estimate Lab values, the scanner 40 can performcolorimetry with high accuracy and high efficiency.

Furthermore, since the estimate Lab value corresponding to the commoncolor patch is determined by performing smoothing processing to theestimate Lab values obtained from the common color patch and the patcheswhich have colors close to the common color patch in the second patchregions 52, stable reading data can be obtained from the scanner 40, andthe reading accuracy can be improved.

In the second patch regions 52, the common color patch and the patcheswhich have colors close to the common color patch may be arranged inaccordance with the degree of color change such as the order of density.However, the positions are not limited as long as the patches arelocated within the second patch regions 52. That is, patches which havephysically similar colors may not be necessarily arranged to be close toeach other, and the reading data of the patches which are arranged atrandom positions may be extracted to be used in the smoothingprocessing.

The description in the above embodiments is an example of the imageforming apparatus according to the present invention, and the presentinvention is not limited to this. Changes can be appropriately madewithin a scope of the present invention for the detailed configurationsand the detailed operations of the sections forming the apparatus.

For example, instead of performing the above-mentioned calibration ofthe scanner 40 in the image forming apparatus, reading data may beobtained from the colorimeter 30 and the scanner 40 to calculate thecorrection values of the estimate Lab values in an external computerapparatus such as a server.

Though the above embodiments have been described by taking, as anexample, the Lab value as the first color value which is output from thecolorimeter 30, the first color value may be the XYZ value, the densityvalue and such like.

Although embodiments of the present invention have been described andillustrated in detail, it is clearly understood that the same is by wayof illustration and example only and not limitation, the scope of thepresent invention should be interpreted by terms of the appended claims.

What is claimed is:
 1. An image forming apparatus that prints an adjustment pattern on a sheet, the adjustment pattern being for correcting a color value obtained by reading a second patch region on the sheet with a second reader based on a color value obtained by reading a first patch region on the sheet with a first reader, the apparatus comprising: a generator that generates image data of the adjustment pattern; and an image former that performs printing on the sheet based on the image data, wherein the adjustment pattern includes a first patch printed in the first patch region and a second patch printed in a region other than the first patch region in the second patch region, the second patch includes a common color patch in a same color as a color of the first patch, and a size of the first patch is larger than a size of the second patch.
 2. The image forming apparatus according to claim 1, wherein the second patch region is wider than the first patch region.
 3. The image forming apparatus according to claim 1, wherein the second patch region includes a plurality of common color patches in the same color, each of the common color patches being the common color patch.
 4. The image forming apparatus according to claim 3, wherein the first patch region is a partial region in a sheet width direction, second patch regions are on both sides of the first patch in the sheet width direction, each of the second patch regions being the second patch region, and each of the second patch regions on the both sides includes the common color patch in the same color.
 5. The image forming apparatus according to claim 1, wherein the first reader and the second reader are disposed on a conveyance path of the sheet printed by the image former.
 6. The image forming apparatus according to claim 5, further comprising a hardware processor, wherein the first reader outputs a first color value, the second reader outputs a second color value, and the hardware processor obtains an estimate first color value by converting a second color value of the common color patch in the second patch region which is output by the second reader into a first color value, the hardware processor obtains a correction value of the estimate first color value for each common color patch by comparing a first color value of a patch in the first patch region which is output by the first reader with the estimate first color value obtained from the common color patch in the same color as the color of the patch in the first patch region, and the hardware processor corrects the estimate first color value obtained by converting a second color value of a color other than the color of the common color patch based on the correction value of the estimate first color value of the common color patch.
 7. An image forming apparatus, comprising: an image former, which forms an adjustment pattern on a sheet; a first reader, which is disposed on a conveyance path of the sheet and outputs a first color value by reading the adjustment pattern, and a second reader, which is disposed on the conveyance path of the sheet and outputs a second color value that is different from the first color value by reading the adjustment pattern; and a hardware processor, wherein the first reader reads a partial region in a sheet width direction, the second reader is configured to read at least a region other than the partial region in the sheet width direction, the adjustment pattern has a first patch region in which a patch that is readable by the first reader is arranged and a second patch region in which a patch that is readable by the second reader in the region other than the region read by the first reader is arranged, wherein the patch in the first patch region is larger than the patch in the second patch region, the hardware processor obtains an estimate first color value corresponding to a color of each patch in the first patch region by converting a second color value of the patch in the second patch region which is output by the second reader into a first color value, the hardware processor obtains a correction value of the estimate first color value for each color of the patch in the first patch region by comparing a first color value of the patch in the first patch region which is output by the first reader with the estimate first color value obtained from the patch in the second patch region, and the hardware processor corrects the estimate first color value obtained by converting a second color value obtained from each patch in the second patch region based on the correction value of the estimate first color value.
 8. The image forming apparatus according to claim 7, wherein the second patch region includes at least a common color patch which is a patch in a same color as a color of the patch included in the first patch region, the hardware processor obtains an estimate first color value corresponding to the color of each patch in the first patch region by converting a second color value of the common color patch in the second patch region which is output by the second reader into a first color value.
 9. The image forming apparatus according to claim 8, wherein patches in the second patch region are randomly arranged, and the second patch region includes a plurality of patches in a same color for the common color patch, and the hardware processor obtains the correction value of the estimate first color value corresponding to the common color patch based on an average value of estimate first color values which are obtained from the patches in the same color for the common color patch in the second patch region.
 10. The image forming apparatus according to claim 8, wherein the hardware processor determines the estimate first color value corresponding to the common color patch by performing smoothing processing to estimate first color values which are obtained from the common color patch and a patch in a color close to a color of the common color patch in the second patch region, and obtains the correction value of the estimate first color value corresponding to the common color patch based on the estimate first color value after the smoothing processing.
 11. The image forming apparatus according to claim 8, further comprising: a density irregularity corrector which corrects density irregularity in the image former, wherein the image former applies the density irregularity corrector when the image former forms the adjustment pattern.
 12. The image forming apparatus according to claim 8, further comprising: a color corrector which calculates a color correction parameter based on the corrected estimate first color value and corrects a color of an image to be formed by the image former based on the color correction parameter.
 13. The image forming apparatus according to claim 8, wherein at least one patch around the common color patch in the second patch region is a patch in a color which is different from the color of the common color patch.
 14. The image forming apparatus according to claim 8, wherein the first reader is a colorimeter, and the second reader is a line sensor.
 15. The image forming apparatus according to claim 8, wherein the first color value is a Lab value, an XYZ value or a density value, and the second color value is an RGB value. 